The cost of solar energy per unit Watt is approximately 5-10 times higher than energy from other sources, which include coal, oil, wind, biomass and nuclear. In order to reduce the cost of solar energy generation in photovoltaic systems, it is desirable to make efficient use of the most expensive part of the system, namely the photovoltaic cell (solar cell). Conventionally, this is done by using large light-focusing solar concentrators (parabolic or trough dishes). These devices have several disadvantages, including high investment cost, high maintenance cost, unwieldy shapes, and the necessity of tracking the sun as it crosses the sky: for a review of current state of the art, see Swanson, Progress in Photovoltaics: Research and Applications 8, 93 (2000).
An alternative option that has been the subject of investigations is to use a waveguide that collects the light and transports it to a small photovoltaic cell. Some of these efforts attempted to use holographic means (U.S. Pat. No. 5,877,874) or geometrical optics to redirect the light (see for example T. Uematsu et al Sol Energ Mater Sol C 67, 415 (2001) and U.S. Pat. No. 4,505,264). These attempts were rather unsuccessful, especially for large transport distances because the efficiencies were low or the systems require tracking of the sun or the systems were complex and not suitable for large-scale production or combinations thereof.
Luminescent solar concentrators (LSC) represent another alternative that has been the subject of investigations, predominantly because these systems are easy to produce at low cost and because these systems do not require tracking of the sun. LSCs consist basically of a large glass or polymeric plate, sheet, film, fibre, ribbon, woven or coating which is doped with fluorescent dye molecules. The dyes absorb light of specific wavelengths from the solar light incident upon it, and re-emit the light in all directions at a longer wavelength. A portion of this light is emitted within the critical angle of the supporting waveguide, and is totally internally reflected and transported to the photovoltaic cell. The LSC has the advantage of combining less expensive materials with flexibility (especially when a plastic waveguide is used) without the need of a heat sink or a sun tracking system. A sample system with a different purpose (room lighting) is described in Earp et al, Sol Energ Mat Sol C 84, 411 (2004). At the moment, LSC-systems are not used commercially which is predominantly related to their poor efficiency. This low overall efficiency originates from a high reabsorption of emitted light (limited Stokes Shift of the dye), from a poor efficiency of coupling light into the waveguide and from a poor efficiency in keeping the light within the waveguide.
The present invention aims to remedy these drawbacks of LSC-systems, in particular by providing means of increasing the efficiency with which emitted light is coupled into the waveguide.